This invention relates to the field of data acquisition instrument triggering, and more particularly to the field of prescaling of oscilloscope trigger sources so that higher frequency trigger sources can be used.
The trigger circuits of all data acquisition instruments, including oscilloscopes, have some bandwidth limitation. When the desired trigger source has a frequency that is beyond this bandwidth, prescalers have sometimes been used to reduce this frequency by dividing the trigger pulses by some modulus M. A prescaler is a fast counter with a modulus that is usually small. M input pulses produce one output pulse. Some prescalers permit M to be a variable number, controllable through an auxiliary set of inputs. However, at higher frequencies, currently about 5 GHz, only fixed modulus prescalers are commercially available.
While fixed modulus prescalers are suitable for many data acquisition applications, for some applications the use of a fixed modulus prescaler can create a problem. One such troublesome application can arise in the use of a prescaler in an oscilloscope's triggering circuitry. Prescalers are used at the front end of oscilloscope trigger circuitry to divide a high frequency trigger source down to a frequency within the bandwidth of the rest of the oscilloscope's trigger section. The difficulty can occur when one wishes to use the oscilloscope to view high speed, time-multiplexed signals on a single line, when their occurrence is synchronized to an even higher speed system clock. The oscilloscope operator may wish to view these signals as they are multiplexed together on this single line. Under these circumstances, if the number of sequentially multiplexed signals, N, is related to the fixed prescaler modulus, M, by certain relationships, the oscilloscope will only be triggered while a sub-set of the multiplexed signals are present. This can occur whenever both N and M are divisible by any whole number other than one.
For example, if the number of multiplexed signals is sixteen (N=16) and the fixed prescaling factor is eight (M=8), the oscilloscope will always trigger when only two of the sixteen signals, S0 through S15, are present. For instance, the oscilloscope might only see signals S0 and S8, or only S5 and S13, or only some other pair of the sixteen signals. Which pair will be seen depends on what happened to be present when the high speed system clock was connected to the prescaler. This occurs because the two signals are synchronized to the same source and in an integral relationship with each other, i.e., N and M share a common factor. Since even powers of two are in common use for both M and N, this situation occurs quite frequently. Since the operator of the oscilloscope wishes to view the whole composite waveform, with contributions from all of the signals, this behavior frustrates his intentions.
What is desired is a method and apparatus that will prevent unwanted synchronization from obstructing an instrument operator's view of N multiplexed signals synchronized to a high speed system clock being used to trigger an instrument through a prescaler with a fixed prescaling factor M, where N and M are related by a common factor.